1. Technical Field
The present disclosure relates to a method for cleaning a contact pad of a microstructure or device to be tested when it is in electric contact with a measure apparatus.
The disclosure also relates to a cantilever contact probe and to a testing head comprising a plurality of cantilever contact probes.
2. Description of the Related Art
As it is well known, a testing head is a device being suitable to electrically connect a plurality of contact pads of a microstructure (such as an integrated circuit) with corresponding channels of a testing machine performing the test thereof.
The test performed on integrated circuits is for instance used to detect and isolate defective circuits already in the production step. Generally, testing heads are thus used to electrically test on-wafer integrated circuits before cutting and assembling them within a chip-containing package.
The efficiency and reliability of a measuring test depends, among other factors, to the realization of a good electrical contact between the microstructure and the measure apparatus and, thus, to a good electric contact between probe and pad.
Moreover, the electric connection between measure apparatus and microstructure, while good at the beginning, can substantially worsen over the time and even become null due to the deposition of dirt and to the formation of oxide on the contact pads of the microstructure to be tested.
For ensuring the reliability and efficiency of a testing head, a “cleaning” said contact pads should be provided.
Among the types of testing heads used in the technical field here considered to test integrated circuits, the so-called cantilever-probe testing heads are widely used, which are just probes projecting, like a fishing rod, over a device to be tested.
In particular, a cantilever testing head of the known type usually supports a plurality of probes, being flexible, usually thread-like, having prefixed electrical and mechanical properties. The probes, projecting in a cantilever manner from the testing head, have a substantially hooked structure, due to the presence of an end portion bent as an elbow which has an internal obtuse angle.
As schematically shown in FIG. 1A, a testing head 1 of this known type generally comprises at least a resin support 2, which is for example ring-shaped, being suitable to incorporate a plurality of mobile contact elements or probes 3, generally composed of special alloy wires with good electrical and mechanical properties, coming out through a projecting section thereof 4 from the resin support 2 with a suitable angle with respect to a plane of a device 8 to be tested, commonly called body angle, indicated with α in the enlarged view of FIG. 1B. These slanted probes 3 are commonly indicated with the English term “cantilever” and the related testing heads are called cantilever heads.
In particular, probes 3 have an end portion or hook 5, bending with a suitable angle with respect to a perpendicular to the device 8 to be tested, commonly called impact angle and indicated with β in the enlarged view of FIG. 1B. The hook 5 of each probe 3 ends with a contact tip 6 having a diameter Dt being suitable to abut and contact a plurality of contact pads 7 of the device 8 to be tested.
The term “Resin clearance”, indicated in FIG. 1A with RCL, also identifies the difference between the height of a lower edge of the resin support 2 with respect to the plane of the device 8 to be tested, i.e. the contact pads 7, and a length H of the hook 5 of probes 3, as indicated in FIG. 1B.
The good connection between the probes 3 of the testing head 1 and the contact pads 7 of the device 8 to be tested could be ensured by the pressure of the testing head 1 on the device itself, probes 3 vertically bending (also with respect to the plane defined by the device to be tested) in the opposite direction to the device movement towards the testing head 1.
As schematically shown in FIGS. 2A and 2B with reference to a single probe 3, when the device 8 to be tested vertically moves against the hook 5 and the contact tip 6, the probe 3 bends, the projecting section 4 of the probe 3 thus forming a working arm for the probe itself in the vertical bending movement thereof, commonly indicated with the term “free length”.
The hooked shape of the probes 3 is such as to allow, during the contact with the contact pads 7 of the device 8 to be tested and the travel upward of probes 3 beyond a preset contact point, commonly called “overtravel” and indicated with OT in the enlarged view of FIG. 2B, contact tips 6 of the probes 3 to slide on contact pads 7 over a length commonly called “scrub”, indicated with Sc in the enlarged view of FIG. 2B. In particular, in these Figures, A indicates a first-contact starting position of the probe 3 with a contact pad 7 and B indicates a pressure-contact working position between the probe 3 and the pad 7, with subsequent horizontal movement or scrub of the contact tip 6 on the contact pad 7 and vertical movement or overtravel OT of the probe 3.
In other words, the contact between the probes 3 of the testing head 1 and the corresponding pads 7 of the microstructure or integrated circuit to be tested could be ensured by a suitable mutual approaching of the testing head and said integrated circuit. Due to this approaching, starting from the contact to the pads 7 of the integrated circuit, a lateral flexing of the end portion of the probes, whose free ends, i.e. the contact tips 6, are pressed to move, crawling on the pads 7 themselves, so as to realize the so-called “scrub” of these contact pads 7. To such a crawled movement corresponds a “scraping” or “brushing” of the surface of the contact pads 7, indeed, with a consequent removal of the dirt or oxide layer being in case on them.
The moving or crawled scrubbing of the contact tips 6 on the pads 7 should ensure a lasting optimal electric contact between the probes 3 and the pads 7 of the device 8 to be tested.
The correct working of a testing head is thus basically linked to two parameters: the vertical movement, or overtravel OT, of probes 3 and the horizontal movement, or scrub Sc, of contact tips 6 of such probes. In particular, a sufficient contact tip scrub on contact pads 7 should be ensured, which allows the pad surface to be “cleaned”, improving the testing head 1 contact. The probes 3 of known testing heads essentially perform a “planning” of a surface of contact pads 7 removing the “dirt” thereon.
This technique, even advantageous when used for integrated circuits of the conventional type which have been used since lately, has an acknowledged bad technical drawback when used for cleaning recently designed integrated circuit. In fact, since some time, integrated circuits (and similar microstructures) are designed and provided being more and more full of contacts pads, the pads being smaller and smaller, for which the above described cleaning technique (removing of the superficial oxide layer) by a scrubbing movement being sufficiently and intentionally long, cause the overcoming of the pad outlines by the contact tips with a consequent loss of electrical contact and a risk of damaging the same device.
In particular, obtaining a sufficient scrub of the contact pads 7 in order to ensure an electrical contact with the device 8 to be tested in all working conditions of the testing head 1 could be in contrast with the present market need urging to design devices being denser and denser with contact pads 7 being smaller and smaller. The movement of contact tips 6 of probes 3 on contact pads 7 allowing a sufficient scrub Sc to be obtained is such that, in devices with reduced-size pads, the contact tip 6 risks going beyond the pad limits, thus subsequently not setting up the electrical connection with the device 8 to be tested and risking damaging the probe 3 or the device itself.